Experimental characterization of bi-directional terahertz emission from gold-coated nanogratings

The THz radiation emission of Au-coated nanogratings (fused silica substrate, 30?nm Au layer thickness, 500?nm grating constant) upon fs laser irradiation (785?nm, 150?fs, 1?kHz,???1?mJ/pulse) is observed in both directions along the laser beam axis (forward and backward) and for both, illumination of the Au/air or the Au/silica interface. THz radiation along the laser beam propagation is emitted in a narrow solid angle of about 15°?fwhm independent on the laser pulse fluence, the angle of incidence and the nanograting profile. The bar width and groove depth of the nanograting as well as the angle of laser beam incidence strongly affect the THz radiation yield. The energy of single THz light pulses is measured absolutely (2?fJ in the 0.3?C0.38?THz range) using a highly sensitive and fast superconducting transition edge sensor. The bi-directional emission of THz radiation is in agreement with the model assumption of surface plasmon polaritons propagating simultaneously on both Au layer interfaces (Au/air and Au/silica).     

超导Tl2Ba2CaCu2O8+x 薄膜的太赫兹复电导率的提取

采用低温太赫兹时域光谱系统, 测试了高温超导 Tl2Ba2CaCu2O8+x 薄膜的太赫兹透射谱, 并提取了它在不同温度下的太赫兹电导率. 研究过程中发现提取后的参数存在电导率随频率波动大、与理论值偏差较大等问题.通过对基片厚度和太赫兹波入射角度的误差对高温超导薄膜电导率的影响进行了分析, 结果表明导致数据波动大是由于基片厚度的偏差引起的. 针对厚度差的影响, 一种矫正方法被提出, 通过对厚度的修正, 提高了数据提取的质量.     

Resonant terahertz generation from InN thin films

Highly efficient conversion from ultrafast optical pulses to their terahertz (THz) counterparts has been achieved with InN thin films. An average THz output power as high as 0.931 microW has been obtained for an average pump power of 1 W, corresponding to a normalized conversion efficiency of 190% mm(-2). Based on our measured dependences of the THz output power on pump polarization, incident angle, pump power, and InN film thickness, resonance-enhanced optical rectification is one of the most plausible mechanisms for the THz generation in the InN films.     

THz wave polarization-controlled spectroscopic imaging for anisotropic materials

We present a polarization-controlled terahertz (THz) wave spectroscopic imaging modality to investigate the anisotropy of the detected materials. The polarization of the emitted THz wave is controlled by changing the relative phase between the fundamental and second-harmonic waves in the two-color laser-induced air plasma THz generation configuration. The THz wave polarization direction is extracted by measuring the two electric field amplitudes when the polarization of the incident wave is controlled to be horizontal and vertical. The anisotropy of the industrial Sprayed-On-Foam-Insulation (SOFI) is characterized by measuring its azimuthal angle dependent THz polarization response. This work demonstrates that THz wave polarization-controlled imaging technique can be used for highly sensitive industrial nondestructive inspection and biological related characterization.     

Voigt位型下电介质/反铁磁/电介质结构二次谐波生成非倒易性研究

利用非线性传递矩阵方法研究了Voigt位型下电介质/反铁磁/电介质 结构二次谐波生成的非倒易性. 研究发现外加静磁场反向和电介质层排序翻转均对二次谐波输出产生影响, 出现了二次谐波生成的非倒易性. 二次谐波生成非倒易性频率区域在反铁磁共振区, 此区间正处于THz频段. 随着入射角度的增加, 非倒易性的效果越来越明显. 研究二次谐波生成的非倒易性, 可为反铁磁器件的设计加工提供理论支持.     

基于IrMn/Fe/Pt交换偏置结构的无场自旋太赫兹源

与目前商用的太赫兹源相比,自旋太赫兹源具有超宽频谱、固态稳定以及成本低廉等优点,这使其成为下一代太赫兹源的主要研究焦点.但使用自旋太赫兹源时,通常需要外加磁场使铁磁层的磁化强度饱和,才能产生太赫兹波,这制约了其应用前景.基于此,本文制备了一种基于Ir Mn/Fe/Pt交换偏置结构的自旋太赫兹波发生器,通过Ir Mn/Fe中的交换偏置场和Fe/Pt中的超快自旋流注入与逆自旋霍尔效应相结合,在无外加磁场下产生了强度可观的太赫兹波.在Ir Mn和Fe的界面中插入超薄的Cu,可以使Fe在厚度很薄时零场下实现饱和磁化,并且其正向饱和场最高可达–10 m T,从而进一步提升无场下的太赫兹发射效率.零场下出射的太赫兹波的动态范围超过60 d B,达到可实用化的水平.通过旋转样品,发现产生的太赫兹波的偏振方向也会随之旋转,并且始终沿着面内垂直于交换偏置场的方向.此外,在此交换偏置结构的基础上,引入了一层自由的铁磁金属层Fe,设计了一种以Ir Mn/Fe/Pt/Fe为核心结构的自旋阀太赫兹源,发现产生的太赫兹强度在两层铁磁层反平行排列时比平行排列以及不引入自由铁磁金属层时均大约提升了40%.结果表明,基...     

Single-shot spatiotemporal measurements of high-field terahertz pulses

The electric field profiles of broad-bandwidth coherent terahertz (THz) pulses, emitted by laser-wakefield-accelerated electron bunches, are studied. The near-single-cycle THz pulses are measured with two single-shot techniques in the temporal and spatial domains. Spectra of 0-6 THz and peak fields up to approximately or = 0.4 MV cm(-1) are observed. The measured field substructure demonstrates the manifestation of spatiotemporal coupling at focus, which affects the interpretation of THz radiation as a bunch diagnostic and in high-field pump-probe experiments.     

Properties of terahertz wave generated by the metallic carbon nanotube antenna

The properties of terahertz (THz) radiation pulses emitted by a metallic, large aspect ratio carbon nanotube antenna have been studied both in the THz waveforms and field distribution. The peak THz field up to 2.66 and 1.26 kV/cm are observed at the probe points. The proposed antenna is designed to operate for dual frequency applications from 2.36 to 2.58 THz and from 7.27 to 7.5 THz for less than -10 dB return loss.     

激光等离子体光丝中太赫兹频谱的调控

研究了双色激光场激发空气成丝产生太赫兹辐射频谱的变化规律.实验观察到随驱动光功率和光丝长度增加,太赫兹光谱主要发生红移的现象.分析表明,由于等离子体密度的增加,太赫兹辐射的趋肤深度减小,等离子体吸收主导了红移的发生.在光丝足够短的条件下,趋肤深度远大于光丝长度,从而产生等离子体振荡主导的太赫兹辐射光谱蓝移.本研究为超快宽带太赫兹辐射的频谱调控提供了新思路.     

光电导开关产生太赫兹电磁波双极特性分析

利用光电导体产生太赫兹电磁波(THz波)，THz远场辐射波形与光电导体材料的载流子寿命、偏置电场以及触发光有直接关系.用不同方法对低温GaAs(LT-GaAs)和半绝缘GaAs(SI-GaAs)光电导开关辐射的THz电磁波所呈现的双极特性进行了模拟计算.结果表明，LT-GaAs光电导开关辐射THz波呈现双极性的主要原因是光生载流子寿命小于一个THz波产生时间；而光生载流子寿命大于100ps的SI-GaAs光电导开关，在不同的实验条件（不同偏置电场、不同光脉冲能量）下，产生的THz波呈现双极特性的主要原因分别是载流子发生了谷间散射和空间电荷电场屏蔽. 关键词： 光电导开关 THz电磁波 载流子寿命 空间电荷屏蔽     

红外低发射率隐身涂层对太赫兹波的反射光谱研究

为研究红外低发射率隐身涂层对太赫兹波的反射特性,制备了红外低发射率隐身涂料,测试了其可见光效果、红外热像图及红外发射率等特性参数。以土黄色红外低发射率涂料为测试样品,利用透射式太赫兹时域光谱系统获得了样品在太赫兹波段的复折射率。分析了特征矩阵理论,并利用特征矩阵理论计算了涂层厚度(0.3~0.5 mm）与入射角度(0°~60°)的变化对入射太赫兹波反射特性的影响。结果表明,在相应厚度及入射角度范围内,太赫兹波在0.8 THz频率下具有多个反射峰值,最高值可达90%以上,有利于实现太赫兹波对红外低发射率隐身涂层下金属目标的探测。此外,涂层厚度变化对入射太赫兹波反射率具有较大影响,涂层越厚,太赫兹波的反射振荡越多,反射峰值越大。入射角度对太赫兹波的反射特性具有一定的影响,但整体影响不大,有利于太赫兹波实现多角度目标的探测。最后,以表面均匀涂覆0.42 mm厚涂料的金属板为测试样品,实验测量了样品在0.1~1.5 THz频率范围内的反射特性,并与部分理论计算结果进行对比。结果表明：实验测量结果与理论计算结果在数值和趋势上较为吻合,但也存在一定的偏差。究其原因,主要由样品厚度和样品参数误差导致,但依然可利用特征矩阵理论研究红外低发射率涂层对太赫兹波的反射光谱特性。     

Propagation characteristics of oblique incidence terahertz wave through non-uniform plasma

The propagation characteristics of oblique incidence terahertz(THz) waves through non-uniform plasma are investigated by the shift-operator finite-difference time-domain(SO-FDTD) method combined with the phase matching condition.The electron density distribution of the non-uniform plasma is assumed to be in a Gaussian profile. Validation of the present method is performed by comparing the results with those obtained by an analytical method for a homogeneous plasma slab.Then the effects of parameters of THz wave and plasma layer on the propagation properties are analyzed. It is found that the transmission coefficients greatly depend on the incident angle as well as on the thickness of the plasma, while the polarization of the incident wave has little influence on the propagation process in the range of frequency considered in this paper. The results confirm that the THz wave can pass through the plasma sheath effectively under certain conditions,which makes it a potential candidate to overcome the ionization blackout problem.     

Terahertz radiation by an ultrafast spontaneous polarization modulation of multiferroic BiFeO3 thin films

Terahertz (THz) radiation has been observed from multiferroic BiFeO3 thin films via ultrafast modulation of spontaneous polarization upon carrier excitation with illumination of femtosecond laser pulses. The radiated THz pulses from BiFeO3 thin films were clarified to directly reflect the spontaneous polarization state, giving rise to a memory effect in a unique style and enabling THz radiation even at zero-bias electric field. On the basis of our findings, we demonstrate potential approaches to ferroelectric nonvolatile random access memory with nondestructive readability and ferroelectric domain imaging microscopy using THz radiation as a sensitive probe.     

Observation of ultra-broadband terahertz emission from ZnTe films grown by metaloganic vapor epitaxy

ZnTe films were grown on (0001) sapphire substrates by the metalorganic vapor phase epitaxy (MOVPE) method. Single crystalline (111) ZnTe epitaxial layers were confirmed by x-ray diffraction, reflection high-energy electron diffraction, and cathodoluminescence measurements. Emission of THz radiation with a spectral distribution up to 40 THz was clearly observed from the ZnTe film with a thickness of 10 μm. The results show that MOVPE is a promising growth method for obtaining high-quality ZnTe epitaxial films on sapphire substrates, which paves the way for obtaining thinner ZnTe films to provide a flatter frequency response in THz device applications.     

Coherent control of THz wave generation in ambient air

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.     

太赫兹波光谱特性分析

目前太赫兹技术的研究主要集中在它的产生、探测机理研究上。由于太赫兹波处于微波和可见光之间的频率范围,已有的微波和光波理论是否能适用于太赫兹波或者具有某些共同的特性仍需实验验证。通过实验分析验证了太赫兹波在空气介质中在垂直于传播方向的平面内场振幅是服从高斯函数分布的,测量给出了太赫兹波的能量分布图。根据测试数据推导出太赫兹波在空气介质中能量衰减公式,利用法布里-珀罗(F-P)干涉仪原理设计出太赫兹波长仪,对美国Corehent公司SIFIR-50THz太赫兹激光器发射的1～3THz波长进行了测量。讨论分析了远场发射角、光束入射角度、机械振动、温度波动和折射率n波动等相关因素对测量精度的影响。     

Growth of thin Fe(0 0 1) films for terahertz emission experiments

The electrical and magnetic properties of thin iron (Fe) films have sparked significant scientific interest. Our interest, however, is in the fundamental interactions between light and matter. We have discovered a novel application for thin Fe films. These films are sources of terahertz (THz) radiation when stimulated by an incident laser pulse. After intense femtosecond pulse excitation by a Ti:sapphire laser, these films emit picosecond, broadband THz frequencies. The terahertz emission provides a direct measure of the induced ultrafast change in magnetization within the Fe film. The THz generation experiments and the growth of appropriate thin Fe films for these experiments are discussed. Several criteria are used to select the substrate and film growth conditions, including that the substrate must permit the epitaxial growth of a continuous, monocrystalline or single crystal film, yet must also be transparent to the emitted THz radiation. An Fe(0 0 1) film grown on the (0 0 1) surface of a magnesium oxide (MgO) substrate makes an ideal sample. The Fe films are grown by physical vapor deposition (PVD) in an ultrahigh vacuum (UHV) system. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) are used to characterize the Fe(0 0 1) films. Two substrate surface preparation methods are investigated. Fe(0 0 1) films grown on MgO(0 0 1) substrates that are used as-received and films grown on MgO(0 0 1) substrates that have been UV/ozone-cleaned ex vacuo and annealed in vacuo produce the same results in the THz generation experiments. Either substrate preparation method permits the growth of samples suitable for the THz emission experiments.     

Nonlinear cross-phase modulation with intense single-cycle terahertz pulses

We have demonstrated nonlinear cross-phase modulation in electro-optic crystals using intense, single-cycle terahertz (THz) radiation. Individual THz pulses, generated by coherent transition radiation emitted by subpicosecond electron bunches, have peak energies of up to 100 microJ per pulse. The time-dependent electric field of the intense THz pulses induces cross-phase modulation in electro-optic crystals through the Pockels effect, leading to spectral shifting, broadening, and modulation of copropagating laser pulses. The observed THz-induced cross-phase modulation agrees well with a time-dependent phase-shift model.     

Terahertz response and colossal Kerr rotation from the surface states of the topological insulator Bi2Se3

We report the THz response of thin films of the topological insulator Bi2Se3. At low frequencies, transport is essentially thickness independent showing the dominant contribution of the surface electrons. Despite their extended exposure to ambient conditions, these surfaces exhibit robust properties including narrow, almost thickness-independent Drude peaks, and an unprecedentedly large polarization rotation of linearly polarized light reflected in an applied magnetic field. This Kerr rotation can be as large as 65° and can be explained by a cyclotron resonance effect of the surface states.     

Tunable pulse delay in an anisotropic metamaterial slab

We show that large and tunable pulse delays can be obtained at propagation through an anisotropic metamaterial slab. The pulse delay depends not only on frequency but also on the incident/propagation angle and polarization of the electromagnetic field, the last two parameters being much easier to tune than the frequency of the incident field. Although there is a trade-off between large pulse delay values and large tuning ranges, the pulse delay can be modified several times by changing the incidence angle. The results apply to a wide frequency range, from the visible to the THz spectrum.